Genes. Inherited characteristics that are either physically displayed or involved in the innner workings of the body. Your DNA defines you. What is contained in your genetic material may in fact decide for you if you will be a marathon runner or a power lifter.
Sara C.P. Williams wrote an article for Science NOW online magazine that shows that a MISSING gene in lab rats led to those rats running six times longer than the control group on a nightly basis.
The University of Pennsylvania conducted the research and found that the abscence of the IL-15Ra gene seemed to change the animals on the cellular level, in which the cells contained more mitochondria than normal. Mitochondria is the power house of the cell and converts through oxidation glucose, pyruvate, and NADH into ATP(muscle fuel). SO more mitochondria = more muscle GO JUICE!
And check this out...later they went back and studied the genetic material of former Olympic athletes(yes there is a database) and found that marathon runners and other endurance athletes had at least one type of variant of the IL-15Ra gene.
Please do not credit me for this. The original article is here at SCIENCE NOW ONLINE MAGAZINE